# Variables and Expressions

Narrated by Horst

```draw_tree(225, 35)
draw_tree(420, 45)
draw_house(720, 60)
draw_snow_person(300, 20)```
• How to use variables (this chapter)

• How to write expressions (this chapter)

• How to create our own functions (the next two chapters)

# Variable and Function Names

## Good Variable Names

• temperature_in_celsius

• tree_position_1

• tree_position_2

• car_speed

• number_of_children

• simpson

## Bad Names, that still work:

• temperatueInCelsius - Uses capital letters. Keep it lower case and use underscores.

• x - Too short, and not descriptive.

• Smith - Starts with a capital letter.

## Names that don't work:

• tree position - Can’t use spaces

## Constants

• Variables that don't change are constants

• They should be in all upper-case

• PI = 3.14159

• SCREEN_WIDTH = 800

# How to Create Expressions

Operator Description
- Subtraction
* Multiplication
** Exponentiation
/ Division
// Integer division
(always rounds down)
% Modulus
(division remainder)

# Using Operators in Expressions

## What's Different?

• Juxtaposition doesn't work - It doesn't multiply
• The '=' isn't an algebraic equality - It is an assignment

# Assignment Operators

Operator Description
= Assignment
+= Increment
-= Decrement
*= Multiply
/= Divide

# Order of Operations

`average = 90 + 86 + 71 + 100 + 98 / 5`
`average = (90 + 86 + 71 + 100 + 98) / 5`